fix: TUI always launches CubApp (infallible init), depresolve module, cub binary unified

- lib.rs: removed fallback help screen, always launches ratatui CubApp
- app.rs: CubApp::new() infallible — fallback to /tmp/.cub if HOME missing
- app.rs: AUR client graceful fallback (None on error or AUR_OFFLINE)
- storage.rs: from_root() made pub for fallback store construction
- depresolve.rs: yay-style topological sort + provider + circular detection
- cubl→cub: unified binary name, OS detection via cfg!(target_os)
- 69 tests pass, 0 failures, clean build

local/recipes/system/cub/source/cub-lib/src/storage.rs

@@ -102,7 +102,7 @@ impl CubStore {
         self.recipes_dir().join(name).is_dir()
     }
 
-    fn from_root(root_dir: PathBuf) -> Self {
+    pub fn from_root(root_dir: PathBuf) -> Self {
         Self { root_dir }
     }
 }